The invention relates to a display device, particularly a display device comprising an electro-optical medium between two transparent, parallel supporting plates provided with a pattern of pixels, said device having a light element.
The invention also relates to a display cell for use in such a display device.
Such display devices are used, for example, in (projection) colour television or in colour monitors for data display in computer systems or in display devices in dashboards, etc.
In British Patent Specification No. 2,154,355, a liquid crystal display device composed of a liquid crystalline material between two glass plates for operation in the transmission mode. The includes a pattern of UV-sensitive phosphors and means for separately switching areas associated with this pattern.
In operation, ultraviolet radiation emitted by a UV-source causes conversion in the luminescent layers to a colour in the visible part of the spectrum (for example to the primary colours red, green and blue), and that visible light is passed on or not passed on by the electro-optical medium according to (colour) picture information.
However, during this conversion a large part of the quantity of light generated in the phosphors is lost. In fact, the conversion is realized within a very thin layer (approximately 2 to 3 microns) on the side of the incident ultraviolet radiation.
To avoid the division into separate luminescent areas, a luminescent layer emitting uniformly distributed light is coated with a colour filter in another embodiment.
Apart from the aforementioned drawback of light absorption in the luminescent layer this solution, similarly as in conventional liquid crystal display devices of the transmission type, has the drawback that the greater part of the light is absorbed in the colour filters. When using an RGB-pattern (red-green-blue) a red filter will pass the greater part of the red light, but will absorb the blue and the green light. Similar remarks apply to blue and green filters. This imposes stringent requirements on the light source in conventional devices.